So? They can remove the packages again after the build is complete (makepkg -r or preferably use devtools) and it's not like this is a another AUR package that requires like a 20 hour build and can only be done on a machine with at least 42 cores and 5TB of ram. It just requires a download and some more disk space which seems like a perfectly fine tradeoff if the alternative is to maintain two PKGBUILDs (one for each python version).
I'm fine with this solution. I wanted to know if there was a better way. So I'll go with this, thx!
Building only certain split packages is no longer supported since commit e8deba3b87784ca14c9afc908046f36a3ad7578c. This will be part of the next pacman version >=4.3 so even if it was possible this would be pointless.
Good to know, I don't follow archlinux development enough to notice this kind of changes. Thanks for your answers, Fabien